Mass spectrometers are used extensively in the scientific community to measure and analyze the chemical compositions of substances. In general, a mass spectrometer is made up of a source of ions that are used to ionize neutral atoms or molecules from a solid, liquid or gaseous substance, a mass analyzer that separates the ions in space or time according to their mass or their mass-per-charge ratio, and a detector. Several variations of mass spectrometers are available, such as magnetic sector mass spectrometers, quadrupole mass spectrometers, and time-of-flight mass spectrometers.
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The magnetic sector mass spectrometer uses a magnetic field or combined magnetic and electrostatic fields to measure the ion mass-per-charge ratio. In one type of magnetic sector geometry, {see A. O. Nier, A Mass Spectrometer for Isotope and Gas Analysis, Review of Scientific Instruments, Vol. 18, No. 6, June 1947, p. 398; L. Holmlid, Mass Dispersion and Mass Resolution in Crossed Homogeneous Electric and Magnetic Fields: The Wien Velocity Filter as a Mass Spectrometer, International Journal of Mass Spectrometry and Ion Physics, Vol. 17 (1975) p. 403} only one mass-per-charge species is detected at any one time, so the magnetic field strength and, if present, the electric field strength must be varied in order to obtain a mass spectrum comprising multiple mass-per-charge species. Major limitations on this type of mass spectrometer are the high mass of the magnet and the time that is required to scan the entire mass range one mass at a time.
Another type of magnetic sector mass spectrometer creates a monoenergetic beam of ions, which are spatially dispersed according to mass-per-charge ratio, and which are focused onto an imaging plate. While this type of spectrometer can detect multiple mass-per-charge species can be detected simultaneously, the poor spatial resolution it provides limits its use to a narrow mass range.
Quadrupole mass spectrometers utilize a mass filter having dynamic electric fields between four electrodes. These fields are tailored to allow only one mass-per-charge ion to pass through the filter at a time. Major limitations of quadrupole mass spectrometers are the high mass of mass of the required magnet and the time required to scan the entire mass range one mass at a time.
Time-of-flight mass spectrometers (TOFMS) can detect ions over a wide mass range simultaneously {see W. C. Wiley and I. H. McLaren, Time-of-Flight Mass Spectrometer with Improved Resolution, Rev. Sci. Instrum., Vol. 26, No. 12, Dec. 1955, p. 1150. Mass spectra are derived by measuring the times for individual ions to traverse a known distance through an electrostatic field free region. In general, the mass of an ion is derived in TOFMS by measurement or knowledge of the energy, E, of an ion, measurement of the time, t1, that an ion passes a fixed point in space, P1, and measurement of the later time, t2, that the ion passes a second point, P2, in space located a distance, d, from P1. Using a ion beam of known energy-per-charge E/q, the time-of-flight (TOF) of the ion is tTOF=t2−t1, and by the ion speed is v=d/tTOF. Since E=0.5 mv2, the ion mass-per-charge m/q is represented by the following equation:
                              m          q                =                                            2              ⁢                              Et                TOF                2                                                    qd              2                                .                            10      
The mass-per-charge resolution, commonly referred to as the mass resolving power of a mass spectrometer, is defined as:
                                                        Δ              ⁢                                                          ⁢                              m                /                q                                                    m              /              q                                =                                                    Δ                ⁢                                                                  ⁢                E                            E                        +                          2              ⁢                                                Δ                  ⁢                                                                          ⁢                                      t                    TOF                                                                    t                  TOF                                                      +                          2              ⁢                                                Δ                  ⁢                                                                          ⁢                  d                                d                                                    ,                    11      where ΔE, ΔtTOF, and Δd are the uncertainties in the knowledge or measurement of the ion's energy, E, time-of-flight, tTOF, and distance of travel, d, respectively, in conventional time-of-flight spectrometers.
In a gated TOFMS in which a narrow bunch of ions is periodically injected into the drift region, uncertainty in tTOF may result, for example, from ambiguity in the exact time that an ion entered the drift region due to the finite time, Δt1, that the gate is “open,” i.e. Δt1≈ΔtTOF. The ratio of ΔtTOF/tTOF can be minimized by decreasing ΔtTOF, for example, by decreasing the time the gate is “open.” This ratio can also be minimized by increasing tTOF, for example, by increasing the distance, d, that an ion travels in the drift region. Often, a reflectron device is used to increase the distance of travel without increasing the physical size of the drift region.
Uncertainty in the distance of travel, d, can arise if the ion beam has a slight angular divergence so that ions travel slightly different paths, and, therefore, slightly different distances to the detector. The ratio of Δd/d can be minimized by employing a long drift region, a small detector, and a highly collimated ion beam.
The uncertainty in the ion energy, E, may result from the initial spread of energies ΔE of ions emitted from the ion source. Therefore, ions are typically accelerated to an energy E that is much greater than ΔE.
A further limitation of conventional mass spectrometry lies in the fact that the source of ions is a separate component from the time-of-flight section of a spectrometer, and it requires significant resources. First, most ion sources are inherently inefficient, so that few atoms or molecules of a gaseous sample are ionized, thereby requiring a large volume of sample and, in order to maintain a proper vacuum, a large vacuum pumping capacity. Second, the ion source typically generates a continuous ion beam that is gated periodically, creating an inefficient condition in which sample material and electrical energy are wasted during the time the gate is “closed.” Third, ions have to be transported from the ion source to the time-of-flight section, requiring, among other things, electrostatic acceleration, steering and focusing. Fourth, typical ion sources introduce a significant spread in energy of the ions so that the ions must be substantially accelerated to minimize the effect of this energy spread on the mass resolving power. Finally, having an ion source separate from the drift region creates an apparatus having large mass and volume.
Still another problem with conventional time-of-flight mass spectrometers is that ions must be localized in space at time t1 in order to minimize Δd and, therefore, minimize the mass resolving power. Typically, time t1 corresponds to the time that the ion is located at the entrance to the drift region.
In summary, the limitations on conventional TOFMS include a mass resolving power dependent on the energy spread of the ions emitted from the ion source; the uncertainty in the distance of travel of the ion in its flight path; the problems associated with an ion source that is separate from the drift region; and the need to localize ions in space at time t1. The present invention provides an apparatus that overcomes these limitations and provides more accurate data.